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A beneficial additive to our diets may be the silent culprit behind plunging sperm production rates in men.

Since the 1950's sperm count and consequently male fertility has been falling at an alarming rate. Many causes have been proposed, including endocrine disruption caused by heavy metals, pesticides and other chemicals in the environment, stress and depression.

But now a group of US researchers have suggested in this week's New Scientist that iodine, found in common table salt, may be the root cause.

Iodine has been added to salt for many years because it has an important role to play in the diet. Iodine is needed by the body to make the thyroid hormone thyroxin, which is vital for brain development. A deficiency can stunt brain growth.

But while iodine is good for the brain, it may be bad for the testicles.

While examining data on sperm levels, the researchers found that the men with the lowest sperm counts would have been born around the time iodised salt was introduced in the American diet, in 1924.

To test the idea, they raised female rats on an iodine-deficient diet and mated them with healthy rats. The offspring were subsequently fed low-iodine diets. To their surprise, the iodine deficient rats' testicles weighed much more and produced more sperm than rats fed a normal diet.

"They ended up with testes twice as big as normal," said researcher James Crissman of the Dow Chemical Company.

Some men with underactive thyroids have also been shown to have enlarged testicles, said co-worker Richard Sharpe of the Medical Research Council's Reproductive Biology Unit in Edinburgh. But he is skeptical, and said that sperm count data can be very variable.

Despite the findings, Crissman warned against cutting down on iodine. "There's a danger people could get exactly the wrong message and decide that sperm counts are more important than brain development," he said.